Magnetic Resonance Imaging (MRI) is a known technique for producing images of inside a body of a patient and is based on the physical phenomenon of magnetic resonance (MR).
During a MR measurement, gradient coils in a gradient coil unit of a magnetic resonance machine are operated with short, high current pulses (e.g., of amplitude 600 Amperes and having rise and fall times of less than one microsecond). The short, high current pulses generate magnetic field gradients used for encoding signals that are essential to producing images.
In addition, typically a main magnet of the magnetic resonance machine generates a main magnetic field that is constant over time and on which the magnetic field gradients are superimposed. In this strong main magnetic field, the conductors of the gradient coils, through which the gradient currents are flowing, experience strong oscillating Lorenz forces. The strong oscillating Lorenz forces cause the gradient coil unit to vibrate and produces a loud audible sound (e.g., >100 dBA) inside the patient receiving zone and the examination room.
This loud audible sound usually causes unease and stress in the patient during the MR measurement (e.g., during prolonged MR pulse sequences). The result is often frustration of the patient and unintentional movements of the patient (e.g., which may reduce the image quality).
The methods for reducing the audible sound reaching the hearing organ of the patient during an MR measurement may be classified into passive and active methods. A passive method uses an additional acoustic barrier to reduce the noise experienced by the patient. These methods attenuate the external sound pressure by ear defenders or earplugs.
Alternative passive methods reduce sounds by damping materials or vacuum-filled sound absorbers used to reduce the inherent vibration of the magnetic resonance machine (e.g., inside the patient receiving zone where the patient is located).
The opportunities for reducing noise using passive methods are rather limited. Although passive methods are normally sufficient for lowering the sound pressure to a level that does not damage the hearing organ of the patient, passive methods are not adequate for preventing the associated unpleasantness for the patient during the MR measurement.
Active methods may supplement passive methods by using destructive interference to attenuate the audible sound further. Active methods may use an external loudspeaker positioned a long way away from the patient receiving zone of the magnetic resonance machine. A plastic tube is used to transfer destructively interfering sound waves from the loudspeaker to the patient receiving zone (e.g., to the ear of the patient).
This method may suffer from interference to the acoustic field and signal delays as a result of the long acoustic path. For high-frequency noise components, especially components above 1 kHz, interference and signal delays may not be eliminated effectively. In addition, the microphones that are normally used to pick up the noise inside the magnetic resonance machine are located so far from the ear of the patient that the noise actually occurring inside the ear of the patient is not picked up. Microphone location may also further limit the noise reduction capability.